Police in the historic Chinese city of Xian have arrested three suspects alleged to have stolen 600 million yuan ($90 million) worth of crypto-currencies. Local media are reporting that the investigation, of what is allegedly China’s biggest-ever Bitcoin heist, started in March after police received information that hackers had taken control of an individual’s computer to steal 100 million yuan ($15 million) worth of crypto-currency.

This led police to follow a “virtual trail”, reports AFP, that brought them to a suspect in the central province of Hunan who, in turn, led them to two more alleged accomplices. Altogether the three suspects are charged with stealing the 600 million yuan by hacking the computers of individuals and companies.

Huashang News reports that the three suspects, who have allegedly been studying hacking since they were aged 13, had all gone on to work for major tech firms. Although the three were taken into custody last week, the police investigation is still under way and now involves law enforcement agencies from three provinces in what is, according to Huashang News, Shaanxi Province’s first virtual currency-related case.

Beijing has, for more than a year, led a series of high-profile crackdowns on crypto crime, ICOs, exchanges and mining operations. According to recent reports from China Money Network citing official Ministry of Justice data, this is causing a growing number of crypto-related legal disputes arising in Chinese courts. At the start of August, Chinese courts were handling 274 legal cases related to crypto-currency.

While many of these cases will be directly linked to Beijing’s attempts to stamp out what it saw as rampant crypto-related fraud, many others, reports China Money Network, result from provincial and district law enforcement agencies finding it hard to clearly define and interpret the central government’s position on the precise legality of crypto trading in China.